In contrast to backlit displays (e.g., a backlit transmissive Liquid Crystal Display, LCD), where light is projected through one or more filters or shutters to create an observable image, a reflective display (e.g., an electrophoretic display, EPD) relies on light reflected off of a reflective surface to generate an image. Typically, reflective displays make use of the ambient light present in the environment where the display is used. Planar front illumination systems have been used for many years to augment the ambient light so that reflective displays can be used in darker environments. Typical planar front light illumination systems are made of clear materials and are attached to the front of reflective electronic displays. Front lights provide supplemental illumination to the face of the display when the reflected ambient light is insufficient to create an observable image.
An ideal front light illumination system would be able to efficiently and uniformly direct the light from a supplemental light source toward the display while not emitting stray light toward the environment or user. This ideal front light illumination system passes all of the reflected light to the user without optical loss or optical artifacts. Further, such an ideal front illumination system would be unobtrusive under ambient lighting, i.e., maintaining the contrast, brightness and image quality of the underlying display. In addition, such an ideal front light is also low cost, thin, lightweight, easily manufactured, compatible with touch technologies and widely available.
One common type of front light illumination system includes a light guide plate constructed with numerous microscopic optical surface features. Each of these optical surface features incrementally redirects a small portion of the light inside the light guide plate using reflection or refraction. Ideally, these optical surfaces extract and distribute the luminous flux within the light guide plate uniformly over the surface of the reflective display. To achieve reflection or refraction without mirrored surfaces (which would be costly and/or difficult to produce), optical engineers carefully construct the critical features and angles of the micro optical surface features to reliably and predictably reflect or refract despite the often poor collimation (i.e., the wide distribution of ray angles) of the source illuminators (e.g., LEDs or fluorescent tubes). The refractive and reflective feature of an optical interface is strongly dependent on the relative indices of refraction of the materials on either side of the interface. To maximize the reflective and refractive power of these micro optical features, the micro optical features are usually exposed directly to air to maximize the refractive index difference.
FIG. 1 shows a front illumination system with microscopic optical surface features. This system comprises a reflective display 100, a light source 101 and a light guide plate 102. The light guide plate has optical features 103 formed on the outer facing surface of the front illumination system. The light source 101 is typically comprised of one or more cold cathode fluorescent lights (CCFLs) or one or more LEDs suitably arranged to produce moderately collimated light 104 directed into a light injection surface of the light guide plate 102.
Common additional features known in the art (not shown) include a reflective housing for the light source, surface treatments on the light source 101 and the injection area of the light guide plate 102, and films or mixing plates inserted between the light source 101 and light guide plate 102 that improve coupling efficiency, uniformity, manufacturability, optical performance and cost. Such additions are applicable to the present invention as well to achieve similar advantageous effects.
The light guide plate 102 has nominally coplanar light guiding surfaces (top and bottom of 102 in FIG. 1). A substantial portion of the light 104 injected into light guide plate 102 remains within the light guide plate 102 due to the well-known optical effect of total internal reflection (TIR). Light guide plate 102 has a plurality of micro optic features 103 on its outer surface that redirect a portion of the guided rays 107 downward at each micro optic feature 103. Ideally, the injected light 104 is uniformly redirected and distributed across the entire surface of the reflective display 100. To achieve uniformity, the density, height, angle, pitch and shape of the micro optic features 103 and the thickness or shape of the light guide plate 102 is modulated across the breadth and width of the light guide plate 102 to account for the diminished light flux as a function of distance from the light source 101.
The incrementally redirected light 107 illuminates the reflective display 100 creating reflected rays 109 that can be seen by a user (the user, not shown, is above the front illumination systems as illustrated herein).
A typical front illumination system is usually only activated when the ambient light 108 falling on the display from external sources is insufficient for the user to perceive an image from the reflective display 100. When ambient illumination 108 is strong enough and consequently the front illumination source is not needed, the front illumination system should be as unobtrusive as possible. Specifically, the front light system should not create unusual reflections, image artifacts or stray light paths that degrade the appearance of the underlying display.
FIG. 2 shows a prior art back illumination system comprising a transmissive display 200, a light source 201 and a light guide plate 202. Light guide plate 202 has printed white dots 203 on the outer surface farthest from the display 200. The light source 200 injects light 204 into the light guide plate 202, which is then substantially guided by total internal reflection in a lateral direction in the light guide plate 202. A plurality of small white dots 203 is screen or inkjet printed, etched, stamped, burned, or molded (among the many conventional methods well known in the art of backlight design) on the outer surface of the light guide plate 202 to act as scattering centers that redirect the guided light in a diffuse scattering pattern 207 towards the transmissive display 200 and ultimately toward the viewer (ray 209). The density, color, scattering effects and/or sizes of the dots 203 is conventionally varied as a function of position to account for non-uniformity of the light source and to compensate for the consumption of guided light flux as a function of distance from the light source 201. As is known in the art, additional films 208 are conventionally placed between the back light and the transmissive display (e.g. diffusers and light redirecting films, polarization recycling films, etc.) to improve the optical efficiency, contrast, viewing angle and uniformity of the overall display.
The refractive and reflective feature of an optical interface between two clear materials (e.g. plastic and air) is strongly dependent on the relative indices of refraction of the materials on either side of the interface. To optimize the light guiding (via total internal reflection) and light extraction (via scattering, reflection or refraction) behaviors, the micro optical features are usually exposed directly to air to maximize the refractive index difference.